The current increase of amino acids production, driven by a synthesis improvement based on microbiological fermentation and chemical or enzymatic processes, demands for probing new sorption media suitable in amino acids separation methods. Indeed, the new challenge is to decrease the amino acids losses that occur during the several adsorption/desorption cycles. Among all the sorbents tested in amino acids sorption (e.g., activated carbons and minerals), synthetic zeolites represent an advantageous class of microporous materials due to their size and shape selectivity, resistance to swelling and thermal stability (Krohn et al., 2005). Although the efficiency of hydrophobic zeolites has been already proved (Munsch et al., 2001), no crystallographic information on the location of those extraframework molecules within the zeolite channels are available. In this contribution, new crystallographic results on two samples of ZSM-5 (MFI topology, Si/Al ratio 21 and 51, respectively) and one sample of L (LTL topology, Si/Al ratio 61) zeolites after adsorption of l-lysine are reported. The sorption process of as-synthesized ZSM-5 and L samples was carried out by means of the batch method (pH=5.5). After contact, kinetics and adsorption isotherms were collected through Capillary Electrophoresis separation technique. X-ray powder diffraction patterns of loaded samples were collected through synchrotron radiation at the MCX Beamline (Elettra, Trieste) at Room Temperature (λ = 0.8270 Å, and 2θ range = 1-65°). Rietveld structural refinements (carried out in the monoclinic and hexagonal symmetry for ZSM-5 and L zeolite, respectively), and processed through the GSAS-EXPGUI software package, highlight on the presence of l-lysine extraframework molecules within zeolite channels (testify through difference Fourier maps analysis). In good agreement with thermal analysis, it results that the pore structure (1-dimensional channel) of L zeolite favours the adsorption of l-lysine. Among ZSM-5 samples, higher volumes of l-lysine compound were detected in SAR 21, thus confirming that the higher the SAR the lower the sorption capacity. These data on ZSM-5 are also corroborated by the comparison of the unit-cell parameters: the higher the l-lysine content the lower the lattice parameter values. This multidisciplinary study allowed to characterize the sorbent structure after l-lysine adsorption and probed the interaction between the sorbent medium and the amino acid molecule. All the information gained suggest that both sorbents tested are particularly performing, albeit L zeolite is more efficient when applied in l-lysine adsorption processes. Krohn, E.J. & Tsapatsis, M. (2005): Amino acid adsorption on zeolite beta. Langmuir., 21, 8743-8750. Munsch, S., Hartmann, M. & Ernst, S. (2001): Adsorption and separation of amino acids from aqueous solutions on zeolites. Chem. Commun. 1978-1979.
ZSM-5 and L zeolites structural characterization after l-lysine amino acid adsorption
Beltrami G.;Pasti L.;Chenet T.;Gigli L.;Martucci A.
2018
Abstract
The current increase of amino acids production, driven by a synthesis improvement based on microbiological fermentation and chemical or enzymatic processes, demands for probing new sorption media suitable in amino acids separation methods. Indeed, the new challenge is to decrease the amino acids losses that occur during the several adsorption/desorption cycles. Among all the sorbents tested in amino acids sorption (e.g., activated carbons and minerals), synthetic zeolites represent an advantageous class of microporous materials due to their size and shape selectivity, resistance to swelling and thermal stability (Krohn et al., 2005). Although the efficiency of hydrophobic zeolites has been already proved (Munsch et al., 2001), no crystallographic information on the location of those extraframework molecules within the zeolite channels are available. In this contribution, new crystallographic results on two samples of ZSM-5 (MFI topology, Si/Al ratio 21 and 51, respectively) and one sample of L (LTL topology, Si/Al ratio 61) zeolites after adsorption of l-lysine are reported. The sorption process of as-synthesized ZSM-5 and L samples was carried out by means of the batch method (pH=5.5). After contact, kinetics and adsorption isotherms were collected through Capillary Electrophoresis separation technique. X-ray powder diffraction patterns of loaded samples were collected through synchrotron radiation at the MCX Beamline (Elettra, Trieste) at Room Temperature (λ = 0.8270 Å, and 2θ range = 1-65°). Rietveld structural refinements (carried out in the monoclinic and hexagonal symmetry for ZSM-5 and L zeolite, respectively), and processed through the GSAS-EXPGUI software package, highlight on the presence of l-lysine extraframework molecules within zeolite channels (testify through difference Fourier maps analysis). In good agreement with thermal analysis, it results that the pore structure (1-dimensional channel) of L zeolite favours the adsorption of l-lysine. Among ZSM-5 samples, higher volumes of l-lysine compound were detected in SAR 21, thus confirming that the higher the SAR the lower the sorption capacity. These data on ZSM-5 are also corroborated by the comparison of the unit-cell parameters: the higher the l-lysine content the lower the lattice parameter values. This multidisciplinary study allowed to characterize the sorbent structure after l-lysine adsorption and probed the interaction between the sorbent medium and the amino acid molecule. All the information gained suggest that both sorbents tested are particularly performing, albeit L zeolite is more efficient when applied in l-lysine adsorption processes. Krohn, E.J. & Tsapatsis, M. (2005): Amino acid adsorption on zeolite beta. Langmuir., 21, 8743-8750. Munsch, S., Hartmann, M. & Ernst, S. (2001): Adsorption and separation of amino acids from aqueous solutions on zeolites. Chem. Commun. 1978-1979.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
